In certain types of hemodynamic systems, a pressure transducer is attached to a pressure dome that is selectively connected by valve means to either of two catheters inserted into the patient's body. In order to reduce the chances of blood particles clogging the catheters, the blood pressure is translated to the transducer by a fluid, usually a saline solution, that fills the pressure dome and the catheters. Further reduction in clogging has been achieved by introducing a slow steady flow of the fluid into the patient's body via the catheters. Even so, it has been found necessary to cause a fast flow or flush of fluid through the catheters from time to time. The means used to introduce the fast flush should be failsafe, i.e., it should permit a fast flush to occur only when it is activated by an operator. Otherwise, enough fluid might flow into the patient to cause injury or into the pressure dome to cause a false alarm of high blood pressure. Heretofore, this function has been performed by an expensive combination of a number of separate valves that can be improperly set so as to connect both catheters to the pressure dome.